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Transcript of These slides relate to the “Using Control Charts in a Healthcare Setting” case. This case was...
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Introduction to Control Charts
Supplement to “Using Control Charts in a Healthcare Setting” case
Variation
Variety may be the “spice of life”, but in the context of product quality, process performance, & service delivery, variation is a BAD thing
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Robbie Gould Image - http://www.nflfootballpicks.org/authors/6/Vic-Busey?Page=9Robbie Gould Stats - http://www.nfl.com/player/robbiegould/2506264/careerstatsGoal Post Image - http://www.partycheap.com/Jointed_Goal_Post_p/50474.htm
Career187 made218 attempts85.8% success rate
CustomerSpecification
Limits
Bears Fans Cheer!
Bears Fans Groan!
Bears Fans Groan!
All Processes are Variable
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“We must understand variation.”W. Edwards Deming
But the key is…
Common Cause Variation• Inherent• Random
Special Cause Variation• Unpredictable• Intermittent
Change
Expected Variation
NewExpected Variation
Good
Variation & Customer Specifications
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Customer Specs
Expected Variation
Customer Specs
Expected Variation
Reacting to Variation
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Tiger Woods Image #1 - http://moodfresher.blogspot.com/2012/02/tiger-woods-wallpapers.htmlTiger Woods Image #2 - http://www.topnews.in/light/people/tiger-woods
One day, Tiger went to the driving range & hit the same way each time.
Not satisfied with the results, he tried again the next day, but tried to improve each shot based upon the last one.
What went wrong?
Over-Reaction to Common Cause Variation?
Examples An operator reacting in the opposite direction based on the previous result Reacting to a single customer complaint without understanding if it is
common to many customers• A manager reacting to a single data point
Setting the current period’s sales quota based on last periods overage or underage
Setting the next period’s budget as a percentage of the last period’s budget The stock market reacting to good news or bad news
Unintended Consequences Existing workers training the new workers
• Symptom of no Standard Work (Lean concept)
Over-reacting to Common Cause Variation leads to MORE variation, not less!
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Understanding Variation
Descriptive Statistics Average Percentage Median Range
Visual Tools Histograms
• An approximation of the distribution’s shape Box Plots Scatter Diagrams Run Charts Control Charts
• SPC – Statistical Process Control
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Run Charts
Displays observed data in a time sequence So what?
3 different run charts with the same distribution
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Control Charts
Run Charts w/Control limits Typically ± 3 standard deviations “Rules” for flagging special causes
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Customer Specs
Expected Variation
Customer Specs
Control Limits & Specification Limits are NOT the same
Some Control Chart Basics
Typically, data is plotted as it happens However, a control chart can be created in an retrospective manner
This approach requires some judgment when determining if there has been a process shift
There are standard control chart tests (see next page) If one of these tests (or conditions) are violated, it is flagged which is
supposed to prompt an investigation as to “why” it happened
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Control Chart Tests
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Attribute
Tests
PlusMore Tests
for
VariableData
Test1
2
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4
5
6
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Numbers of the Line= Failed Test
Control Chart Selection Guide
13Source: Nancy R. Tague’s The Quality Toolbox, Second Edition, ASQ Quality Press, 2005.
Control Chart Selection Guide (another view)
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Chart Type Characteristic MonitoredIndividual X Individual Values
Xbar Sample Mean
R Sample RangeMR Moving RangeD Sample Standard Deviation
Location
Spread
Variable Control Charts
Chart Type Characteristic Monitoredp for defectives (%) – sample size variesnp for defectives (%) – sample size fixedu for defects – sample size fixedc for defects – sample size varies
Attribute Control Charts
More Control Chart Basics
What they can do… Identify a process that is out-of-control Provide statistical evidence when a process has actually changed (either
for better or for worse)
What they cannot do… Identify why the process is out-of-control
• That is left for you to investigate
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Never Stop Asking “Why?”16
Image - http://1.bp.blogspot.com/_IbpK-GhiWSc/TJTfX63zpVI/AAAAAAAACOI/na3ZgyUFd2M/s1600/1227573326bxGKsyD.jpg